1. Field of Invention
The present invention is related to digital cameras and in particular to efficiently determining the automatic focus of a digital camera.
2. Description of Related Art
In digital cameras with auto focus the amount of time that the auto focus routine takes is often critical in being able to capture the desired image. This is particularly important when action or movement of the subject is involved comprising sports events, children at play, animals and other moving objects. The user of an auto focus digital camera can become frustrated when the digital camera refuses to capture a picture because the lens focus hasn't or cannot be found. It is therefore critical that a digital camera auto focus provide a fast and accurate focus of a subject of a picture image.
The focusing of an image can be complicated by the complexity of the image and the lack of definitive portions of the image in a focus box, for instance clear differences between the subject of interest and the background surrounding the subject, as can be the case when performing analysis of an edge for sharpness. Further complicating auto focus is a moving subject where the focus box must be maintained on the moving subject to avoid focusing on the background. Having a fast accurate auto focus method helps mitigate the problems associated with movement of the subject and the frustration of the user of a digital camera.
US Patent Application Publication No. 2009/0136148 (Lim et al.) is directed to a method and apparatus for digital auto-focus, wherein a local block generator creates a local block image from an input image and a distance-estimation unit estimates a distance for each image in the local block. US Patent Application Publication No. 2009/0115886 A1 (Chen) is directed to an auto focus method for a digital camera, wherein a single image is used to obtain a lens focal position by using an edge detection algorithm to compute distance to an object. US Patent Application Publication No. 2009/0102963 A1 (Yeo et al.) is directed to an auto focus image system, which includes an image sensor coupled to a controller that captures an image that has at least one edge with a width, wherein the controller generates a focus signal that is a function of the edge width. In US Patent Application Publication No. 2009/0079862 A1 (Subbotin) a method and apparatus is directed to determining the need to refocus an imaging device using a blur value that determines sharpness. In US Patent Application Publication No. 2009/0074393 A1 (Park et al.) a method and apparatus is directed auto-focus of imaging devices, which includes a distance-calculation based on blur of at least two images.
US Patent Application Publication No. 2009/0060329 A1 (Nakajima et al.) is directed to an image data analyzing device that detects the number of blurred pixels and compares to the total number of pixels to determine the upper size limit of an output image. US Patent Application Publication No. 2008/0226278 (Garg et al.) is directed to multiple sets of pixels wherein each set represents an image with different degrees of focus, whereupon the focus of an image determined by comparing variation of luminance of pixels representing an object. US Patent Application Publication No. 2008/0181595 A1 (Pnueli et al.) is directed to a focusing method that entails receiving a focusing image and shifting the focusing image to obtain a shifted focusing image whereby a focus metric is calculated from the focusing image and the shifted focusing image. US Patent Application Publication No. 2007/0002152 A1 (Fujiwara) is directed to a digital camera that has an edge enhancing section to enhance edges of image data whereby a focus is determined to be good or bad and adjusts the degree of edge enhancement wherein focusing time is shortened.
In U.S. Pat. No. 7,538,815 B1 (Belikov et al) auto-focusing algorithms and implementation are directed to a machine inspection application that can be used with existing imaging hardware without additional components. U.S. Pat. No. 7,389,042 B2 (Lin et al.) is directed to an auto-focus method, wherein there are four lens movement states comprising initial, course, mid, and fine states within each state there are different lens search steps. In U.S. Pat. No. 7,283,663 B2 (Sobel et al.) a method and apparatus is directed to interpolating color image information wherein a processor determines whether there is an edge in the vicinity of a target pixel in which long scale interpolation is performed on image data if there is not an edge in close proximity of the target pixel. U.S. Pat. No. 6,747,808 B2 (Voss et al.) is directed to an electronic imaging device that is focused by capturing data samples of a region of at least one photo detector while changing focus settings and analyzing data samples. In U.S. Pat. No. 6,023,056 (Fiete et al.) a method is directed to an image capture system, wherein a plurality of edges are detected and the sharpness of the edges are measured.